1. Field of the Invention
The present invention relates to a method and apparatus for wireless data transmission between a base station and one or more transponders.
2. Description of the Background Art
Transmission methods between one or more base stations or readers and one or more transponders are used, for example, in contactless identification systems or so-called radio frequency identification (RFID) systems. Sensors, for example, for temperature measurement, can also be integrated into the transponder. Such transponders can also be called remote sensors.
Transponders or their transmitting and receiving devices typically do not have an active transmitter for data transmission to the base station. Such inactive systems are called passive systems when they do not have their own power supply, and semipassive systems when they have their own power supply. Passive transponders derive the energy necessary for their supply from the electromagnetic field emitted by the base station.
Backscatter coupling is typically employed for data transmission from a transponder to a base station with UHF or microwaves in the far field of the base station. For this purpose, the base station emits electromagnetic carrier waves, which are modulated and reflected by the transmitting and receiving device of the transponder corresponding to the data to be transmitted to the base. Typical modulation methods include amplitude modulation, phase modulation, and amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of the subcarrier is modified.
When data transmission between the base station and a transponder occurs synchronously, the base station generates synchronization markers or symbol delimiting markers, which are also called “notches.” In this case, the transponder changes the backscattered signal synchronously to the synchronization markers generated and transmitted by the base station. Furthermore, the synchronization markers can be used for data transmission from the base station to the transponder. The significant value of a symbol transmitted by the base station is hereby, for example, determined by the time interval or a duration between two successive synchronization markers. If the duration is above, for example, a settable threshold, the value of the symbol is “1” or otherwise “0.”
Various methods are known for generating synchronization markers. In general, hereby, the electromagnetic carrier waves or the carrier signal are amplitude- and/or phase-modulated by the base station with a modulation signal. Typically, a delimiting marker is detected in a transponder with use of a so-called RSSI (radio signal strength indicator) circuit.
A method based on amplitude modulation, in which the modulation signal suppresses or hides the carrier signal during a certain modulation time, is the on-off-keying (OOK). The modulated signal, however, has a relatively broadband spectrum. In passive systems, which derive the energy for their supply from the carrier signal, the energy input is also suppressed during the modulation time, as a result of which the achievable range is reduced accordingly. A reduction in the modulation time, during which the carrier signal is hidden, is, however, not arbitrarily possible because the bandwidth required thereby increases.
To increase the transmission range, methods are known, which do not completely hide the carrier signal during the modulation time, i.e., have a modulation index that is less than one. This, however, leads to a decline in transmission security because the delimiting markers can no longer be recognized as certainly as in complete masking.
To reduce the required bandwidth, the modulation signal during the modulation time can have a sinusoidal shape. In other words, the carrier signal is not hidden with a so-called rectangular function, but turned off and again on in a sinusoidal manner.
A form of amplitude modulation is double sideband modulation (DSBM) with a suppressed carrier. The frequency of the carrier signal is suppressed in the resulting spectrum of the modulated signal.
A method is described in FI 20011943, which corresponds to U.S. Publication No. 20040246102, and in which synchronization markers are generated with use of a cosinusoidal modulation signal. The modulation of the carrier signal occurs with use of an IQ modulator, where the modulation signal is applied to the I-input of the IQ modulator and its Q input is kept constant.
Devices in the vicinity of an RFID system, which can also communicate wirelessly, can interfere with data transmission between the base station and the transponders. To reduce the effects of such interference, the frequency of the carrier signal or the modulated carrier signal can be switched or changed between different frequency values. This, for example, reduces the effect of a specific interfering frequency. In some cases, a frequency hopping is also specified by applicable standards.
Because transmission may be made at a specific time only at one frequency, with frequency hopping, the carrier signal must be briefly suppressed before the switching. This suppression causes the RSSI circuit of the transponder to detect a synchronization marker, which can be interpreted as a component of a frame or protocol.
Various scenarios are possible here. If the frequency hopping occurs outside a frame, the synchronization marker evoked by the frequency hopping is interpreted as the beginning of a frame. Due to the absence of a frame no further synchronization markers are received, a receiving unit of the transponder is reset to the initial state after the elapse of a maximum time, because otherwise a communication block would occur. This, however, reduces the possible data throughput.
Another possibility is for frequency hopping to occur within a frame or protocol. If hereby a carrier signal of a first frequency is hidden and a carrier signal of a second frequency is shown, due to the different phase positions of the carrier signals in a transition phase, destructive interference can occur, which is wrongly recognized as a synchronization marker in a transponder. This produces a faulty frame, which can be detected from the presence of a data link layer. If a faulty frame is detected, error processing occurs according to a protocol agreement. As a rule, resynchronization occurs; i.e., the frame is repeated.
In ISO WD 18000-6 mode1 of January 2002, a method is described in which frequency hopping can also occur within a frame, but within a specified, reserved field. Frequency hopping, however, requires resynchronization here as well.
A frequency hopping method is described in WO 97/07413, which corresponds to U.S. Pat. No. 5,905,769, and which uses a switchable synthesizer to switch the carrier frequency. In this case, however, switching times in the millisecond range arise, where no carrier signal is transmitted during the switching times. Because passive transponders are not supplied with power from the electromagnetic field, suitable energy buffers must be present in the transponder and the transmission range is reduced.